1 files changed, 100 insertions, 0 deletions
diff --git a/lib/std/math/trunc.zig b/lib/std/math/trunc.zig
new file mode 100644
index 0000000000..219bcd4914
--- /dev/null
+++ b/lib/std/math/trunc.zig
@@ -0,0 +1,100 @@
+// Ported from musl, which is licensed under the MIT license:
+// https://git.musl-libc.org/cgit/musl/tree/COPYRIGHT
+//
+// https://git.musl-libc.org/cgit/musl/tree/src/math/truncf.c
+// https://git.musl-libc.org/cgit/musl/tree/src/math/trunc.c
+
+const std = @import("../std.zig");
+const math = std.math;
+const expect = std.testing.expect;
+const maxInt = std.math.maxInt;
+
+/// Returns the integer value of x.
+///
+/// Special Cases:
+///  - trunc(+-0)   = +-0
+///  - trunc(+-inf) = +-inf
+///  - trunc(nan)   = nan
+pub fn trunc(x: var) @typeOf(x) {
+    const T = @typeOf(x);
+    return switch (T) {
+        f32 => trunc32(x),
+        f64 => trunc64(x),
+        else => @compileError("trunc not implemented for " ++ @typeName(T)),
+    };
+}
+
+fn trunc32(x: f32) f32 {
+    const u = @bitCast(u32, x);
+    var e = @intCast(i32, ((u >> 23) & 0xFF)) - 0x7F + 9;
+    var m: u32 = undefined;
+
+    if (e >= 23 + 9) {
+        return x;
+    }
+    if (e < 9) {
+        e = 1;
+    }
+
+    m = u32(maxInt(u32)) >> @intCast(u5, e);
+    if (u & m == 0) {
+        return x;
+    } else {
+        math.forceEval(x + 0x1p120);
+        return @bitCast(f32, u & ~m);
+    }
+}
+
+fn trunc64(x: f64) f64 {
+    const u = @bitCast(u64, x);
+    var e = @intCast(i32, ((u >> 52) & 0x7FF)) - 0x3FF + 12;
+    var m: u64 = undefined;
+
+    if (e >= 52 + 12) {
+        return x;
+    }
+    if (e < 12) {
+        e = 1;
+    }
+
+    m = u64(maxInt(u64)) >> @intCast(u6, e);
+    if (u & m == 0) {
+        return x;
+    } else {
+        math.forceEval(x + 0x1p120);
+        return @bitCast(f64, u & ~m);
+    }
+}
+
+test "math.trunc" {
+    expect(trunc(f32(1.3)) == trunc32(1.3));
+    expect(trunc(f64(1.3)) == trunc64(1.3));
+}
+
+test "math.trunc32" {
+    expect(trunc32(1.3) == 1.0);
+    expect(trunc32(-1.3) == -1.0);
+    expect(trunc32(0.2) == 0.0);
+}
+
+test "math.trunc64" {
+    expect(trunc64(1.3) == 1.0);
+    expect(trunc64(-1.3) == -1.0);
+    expect(trunc64(0.2) == 0.0);
+}
+
+test "math.trunc32.special" {
+    expect(trunc32(0.0) == 0.0); // 0x3F800000
+    expect(trunc32(-0.0) == -0.0);
+    expect(math.isPositiveInf(trunc32(math.inf(f32))));
+    expect(math.isNegativeInf(trunc32(-math.inf(f32))));
+    expect(math.isNan(trunc32(math.nan(f32))));
+}
+
+test "math.trunc64.special" {
+    expect(trunc64(0.0) == 0.0);
+    expect(trunc64(-0.0) == -0.0);
+    expect(math.isPositiveInf(trunc64(math.inf(f64))));
+    expect(math.isNegativeInf(trunc64(-math.inf(f64))));
+    expect(math.isNan(trunc64(math.nan(f64))));
+}